January 17, 1919] 



SCIENCE 



73 



obtained from samples of ordinary glass shades, 

 such as are prized by the lovers of stuffed 

 birds. In other words, the cylinder supplies 

 its own slit within, as at r or & in the figure, by 

 refraction. With a narrow beam of sunlight 

 no collimator is needed. 



The spectrum will now be found to be filled 

 with short, slender, horizontal shadows moving 

 endwise in a common direction, but at differ- 

 ent speeds. On pushing the ocular in some- 

 what further, these shadows become sharply 

 defined lines, all nearly horizontal, of all 

 lengths from mere points to black lines half 

 the length of the spectrum or more. On at- 

 tentive observation the black lines are seen to 



be associated with narrow areas of accentuated 

 brightness, so that diffraction patterns are in 

 question. Occasionally a beautiful complete 

 slender spindle-shaped black body with a bril- 

 liant narrow frame of light around it, will ap- 

 pear. Arrowheads holding patches on their 

 notched sides, are not infrequent; but as a 

 whole the spectrum has the appearance of be- 



ing intersected with an interminable array of 

 slow horizontally flying arrows, shot in a com- 

 mon direction from end to end. With regard 

 to the motion, this is more usually horizontally 

 from red to blue; in the lapse of time and in 

 the middle layers always so and not perma- 

 nently from blue to red. Sometimes both mo- 

 tions were seen to occur together in different 

 levels, the retrograde motion being relatively 

 slow, less pervasive and confined to the top or 

 the bottom layers of the liquid. All degrees of 

 speed occurred from a passage through the 

 spectrum in a fraction of a second, to passages 

 lasting over a minute. Under the latter condi- 

 tions it may happen that the particles actually 

 stop and then begin a retrograde movement 

 soon to be accelerated in turn. During this 

 period of transition, particles may be seen also 

 to rise and fall, but with relatively great slow- 

 ness as compared with the usually horizontal 

 motion. Some of the arrows are somewhat 

 oblique to the horizontal. Under rare condi- 

 tions I noticed a line of light instead of 

 shadow. Breadths differ greatly and would 

 naturally depend on focusing. 



Usually the motion persists with apparently 

 undiminished small velocity for hours, so that 

 it much outlasts one's patience. Often a 

 single particle can be observed for a minute or 

 more ; but after 10 or 20 hours all particles dis- 

 appear and the spectrum is clear. From this I 

 concluded that the diffractions are not due to 

 local difference of density, etc., of the solution, 

 as I first supposed, but actually originate in 

 minute solid particles (or in case of other 

 liquids in minute air bubbles) entrapped in 

 the liquid. The slow subsidence and persist- 

 ence of particles indicates this state of things. 



Moreover I found that the initial motion of 

 particles as a whole from red to blue or blue to 

 red, could be controlled by rotating the cylinder 

 G on its axis a, either counter-clockwise or the 

 reverse, respectively. Brownian motions are 

 excluded, since these are promiscuous and 

 since the magnification is inadequate. It is 

 difficult to conceive how the angular momenta 

 impressed on this solution can persist for 

 hours within it, after the solution is apparently 

 quite at rest, even if the solution is of large 



